I hate uppercase!

script.sh

#!/bin/bash

# Go to home directory
cd ~

# Rename all directories
mv Desktop desktop
mv Documents documents
mv Downloads downloads
mv Music music
mv Pictures pictures
mv Public public
mv Templates templates
mv Videos videos

# Update user directories configuration
sed -i 's/Desktop/desktop/g' .config/user-dirs.dirs
sed -i 's/Documents/documents/g' .config/user-dirs.dirs
sed -i 's/Downloads/downloads/g' .config/user-dirs.dirs
sed -i 's/Music/music/g' .config/user-dirs.dirs
sed -i 's/Pictures/pictures/g' .config/user-dirs.dirs
sed -i 's/Public/public/g' .config/user-dirs.dirs
sed -i 's/Templates/templates/g' .config/user-dirs.dirs
sed -i 's/Videos/videos/g' .config/user-dirs.dirs

# Restart your PC!